1. Field of the Invention
This invention relates to a digital receiving apparatus. More particularly, the present invention discloses a novel receiving apparatus for eliminating phase ambiguities which can occur when the carrier reconstruction loop of the receiver is tracking the incoming carrier signal.
2. Description of the Prior Art
When transmitted signals are received which comprise a carrier signal which is phase modulated with digital data, it is not possible to determine the absolute phase of the carrier.
Prior art systems have avoided this problem by transmitting with the modulated carrier signal an unmodulated tone signal. The recovery loop in the receiver can then track the unmodulated tone signal which will enable the determination of the absolute phase of the unmodulated tone signal and the modulated carrier signal.
When the absolute phase of the carrier signal is known, it is possible to coherently detect the received modulated signal. Without knowledge of the absolute phase of the carrier signal, the data received can be correct or inverted; accordingly, it is necessary to provide some means for determining the phase of the carrier.
In the absence of employing an unmodulated tone signal to determine the absolute phase of the carrier signal, the data which is used to modulate the carrier signal at the transmitter may be encoded before being transmitted. Then a decoder may be employed in the receiving apparatus which will permit the data on the carrier to be recovered in a non-inverted mode without determining the phase of the carrier.
If the prior art system employing an unmodulated tone signal is implemented, the power so employed is unusable for data recovery.
If the prior art system employing both differential encoders and differential decoders is implemented, not only is the cost of the additional apparatus incurred, but also any bit error in the receiving apparatus and the detecting apparatus is doubled by the differential decoder. To now obtain the same bit error rate as with a coherent system it is necessary to increase the transmitting power.
The above problem of acquiring and tracking data modulated carrier signals is compounded when a spread spectrum data code is employed. The code tracking and recovery loops of a spread spectrum data code receiver require that non-coherent tracking be employed when the absolute carrier phase is not known. When non-coherent tracking is employed in such spread spectrum receiving systems, there is an increase in the acquisition time of the spread spectrum code.
It would be desirable to provide a coherent spread spectrum receiver which does not require unmodulated tone signals or encoders and decoders while maintaining knowledge of the absolute phase of the data modulated carrier signal.